Fat is a vital organ that our bodies work hard to protect—so instead of battling fat, we should focus on maintaining a healthy level of it

The holidays are long over, but many Americans still have something to remember them by: extra pounds of fat. Our collective effort to fight the bulge makes the first few months of the new year the peak sales period for the U.S. dieting industry. Americans spend a fortune each year trying to get rid of fat, but the flab often creeps back, despite our best efforts. It seems as if our fat has a mind of its own—which, it turns out, isn’t as far-fetched as it sounds.

We usually think of fat as unhealthy excess storage to be gotten rid of at all costs, but researchers have reached a different conclusion: Fat is actually a vital organ that releases essential hormones and sends crucial messages to our brains. And because fat is so important, our bodies work hard to protect it. Instead of simply battling fat, we need to focus on how to maintain a healthy level of it.

Jeffrey Friedman, a molecular biologist at Rockefeller University, was among the first to discover that there was more to fat than just storing calories. In the 1980s, he was researching mice that ate uncontrollably. After nine years, Dr. Friedman discovered that fat produces a hormone that he named leptin (from the Greek leptos, or thin), which is released into the bloodstream and binds with areas of our brain responsible for appetite. His lab’s obese mice had a genetic defect in their fat that prevented them from making functional leptin and getting the signal to stop eating. Humans with a similar genetic defect can eventually eat themselves to death.

Fat’s connection with leptin poses a dilemma: When we lose fat, we also have decreased levels of leptin. As a result, we are hungrier than before weight loss. Leptin also affects our muscles and thyroid hormones, and reduced amounts of it slow down our metabolism. These combined effects of decreased levels of leptin drive us to regain weight.

We now know that fat also can affect brain size. People who are genetically leptin-deficient have smaller brain volume in some areas, as do patients who are malnourished because of anorexia. Leptin also enables wounds to heal faster and strengthens our immune system by activating T-cells.

And leptin isn’t the only crucial hormone produced by our fat. It also manufactures adiponectin, a hormone that keeps our blood clear of harmful toxins and fats.

The benevolent type of fat that is the primary producer of leptin and adiponectin is subcutaneous, found directly under our skin in places such as our abdomens, thighs, buttocks and arms. This should be distinguished from visceral fat, which is stored under the stomach wall, nestled against our internal organs. The latter is the “bad” fat that we hear so much about. It can become inflamed and lead to diabetes and heart disease.

But “good” fat can fight “bad” fat. By making adiponectin, subcutaneous fat guides circulating fats in our blood out of our veins and into the subcutaneous fat tissues where they belong. The hormone also reduces visceral fat. Luckily, exercise promotes the release of adiponectin. This is why sumo wrestlers are both fat and fit (at least until they retire): They exercise around seven hours a day, which helps keep their visceral fat under control even as they pile on the subcutaneous fat that they need to compete.

The new science of fat suggests some health pointers. First, we should appreciate our fat. Our culture is obsessed with shedding flab, as any sports or fashion magazine cover will tell you, but normal amounts help to keep us healthy.

Second, if you’re going to worry, focus on visceral fat, which is associated with disease. For those of us who aren’t sumo wrestlers, jogging 20 miles a week or doing intensive interval training three times a week has been linked with a reduction in visceral fat. Intermittent fasting—prolonging the overnight gap between meals to 14 hours or more—also can help.

Finally, if you are above the average fat mass range of 25-30% for women and 18-24% for men, and you want to maintain your weight loss, it may take more effort than you expect. Fat can alter our appetite and metabolism to drive us to regain weight, an effect that can last for years. So real weight loss requires a long-term effort. By better understanding how fat works, what makes it accumulate (which isn’t just sloth and gluttony) and selecting a diet that will work over the years, we are more likely to succeed at keeping the pounds off.

And as you pick your personal weight targets, remember: Obesity is unhealthy, but too little fat isn’t good for you either.

—Dr. Tara is the author of “The Secret Life of Fat: The Science Behind the Body’s Least Understood Organ and What It Means for You” (Norton).